The present invention relates to a magnetostrictive torque sensor and to an electric power steering apparatus. More particularly, the invention relates to a magnetostrictive torque sensor for detecting a torque, which is applied to a steering shaft and so on, utilizing a magnetic change based on a magnetostrictive action occurring in a magnetostrictive film, and to an electric power steering apparatus utilizing the magnetostrictive torque sensor.
For example, in an electric power steering apparatus equipped as a steering system of an automobile, a steering torque added to a steering shaft from a steering wheel by a driver's steering operation is generally detected by a steering torque detecting portion. In recent years, a device constituted utilizing a magnetostrictive torque sensor has been known as the steering torque detecting portion. The steering shaft is a rotating shaft that receives a torque generated by a driver's steering operation so as to rotate and that functions as a rotating shaft in the steering torque detecting portion. The electric power steering apparatus drive-controls a motor for assisting a steering force in response to a torque signal detected through the steering torque detecting portion. Thus, the electric power steering apparatus reduces a driver's steering force so as to give the driver a comfortable steering feeling.
The principle of detecting a torque, which is employed by a magnetostrictive torque sensor functioning as a steering torque detecting portion, is described below with reference to the accompanying drawings. Actually, the magnetostrictive torque sensor is constituted as a sensor device, which includes an intrinsic sensor part, which is sensitive to a torque, and a detecting electric circuit part that processes a sensor output signal.
In the magnetostrictive torque sensor, magnetostrictive films are formed on a surface of a steering shaft (rotating shaft) 101 over the entire circumference along a shaft center line, for example, magnetostrictive films 102A and 102B are formed at two places thereon so as to have opposite magnetic anisotropies 103 and 104, as illustrated in FIG. 10. The magnetostrictive film 102A has a positive magnetostriction constant, while the magnetostrictive film 102B has a negative magnetostriction constant. The magnetostrictive torque sensor 100 has a sensor configuration enabled so that when an input torque acts upon the steering shaft 101 from a steering wheel, as indicated by arrows 105, a change in the magnetostrictive characteristic of each of the magnetostrictive films 102A and 102B is contactlessly detected by an associated one of detection coils 106A and 106B. The detection coil 106A is disposed around the magnetostrictive film 102A to surround the magnetostrictive film 102A. The detection coil 106B is disposed around the magnetostrictive film 102B to surround the magnetostrictive film 102B.
In the aforementioned torque sensor, a change in the magnetostrictive characteristic of each of the magnetostrictive films 102A and 102B is detected by an associated one of detection coils 106A and 106B. Thus, upon detection of the change, for example, the detection coils 106A and 106B are supplied with sinusoidal wave alternating currents so as to be excited. Excitation sinusoidal wave alternating currents are applied to the detection coils 106A and 106B, respectively. Consequently, alternating magnetic fields are applied to the associated magnetostrictive films 102A and 102B, respectively. Although an example of using the detection coils as excitation coils has been described, the magnetostrictive torque sensor can be configured to use excitation coils separately from the detection coils. In either case, excitation coils for applying alternating magnetic fields to magnetostrictive films are required.
FIG. 11 illustrates the principle of detecting an input torque an input-torque/output according to the configuration of a sensor device that is the magnetostrictive torque sensor 100. A characteristic VT1 is an input torque characteristic produced on the basis of an output signal of the detection coil 106A. A characteristic VT2 is an input torque characteristic produced on the basis of an output signal of the detection coil 106B. The direction of the magnetic anisotropy 103 of the magnetostrictive film 102A is opposite to that of the magnetic anisotropy 104 of the magnetostrictive film 102B. Thus, the direction of inclination of a line representing the characteristic VT1 is opposite to that of inclination of a line representing the characteristic VT2. A characteristic VT3 is an input torque output characteristic produced by calculating the difference between the characteristics VT1 and VT2. An artificial input torque applied to the steering shaft is obtained according to the characteristic VT3. Actually, a point B on a curve representing the characteristic VT3 is set as an origin. A right-side area on the right side of the point B is set as a positive area. A left-side area on the left side of the point B is set as a negative area. According to the characteristic VT3, information on the rotational direction and the magnitude of an input torque to be applied to the steering shaft is obtained.
A conventional magnetostrictive torque sensor is described in Patent Document 1. The magnetostrictive torque sensor described in Patent Document 1 adds an output of a detection coil upon excitation of an excitation coil to a reference signal. The phase of an addition value representing a result of this addition is compared with that of the reference signal. A comparison output representing a comparison result is converted into a voltage. Based on the polarity and the magnitude of the obtained voltage, the direction and the magnitude of a torque to be applied to a torque transmission shaft are detected.
[Patent Document 1] JP-A-2004-191068
In the conventional magnetostrictive torque sensor, a magnetic moment is rotated towards an input direction by a twist torque. A change in the magnetic moment affects the sensitivity of the sensor. Although the sensitivity of the magnetostrictive torque sensor is determined by the product of the magnetostrictive constant and the magnetic permeability of a material, the magnetic characteristic of an initial magnetostrictive film depends upon the material thereof, a manufacturing technique therefor, and a substrate to which the magnetostrictive film is attached. Thus, considerable studies of processes are required to obtain the high-level and less-variation sensitivity of the material, which are determined in the sensor. Variation of the sensitivity of the sensor occurs due to earth magnetism and a disturbance magnetic field caused when a vehicle crosses a railway crossing, or the like, depending upon use environment. However, the processes assuming such a fact have not sufficiently been studied until now.
In a case where the magnetostrictive torque sensor is used as a steering torque detecting portion of the magnetostrictive torque sensor, particularly, highly-functional and less-variation torque sensors are desired. Additionally, a magnetostrictive film having a high signal-to-noise (SN) ratio, which is provided in the magnetostrictive torque sensor, is desired in consideration of high grade steering performance and a threshold margin in a fail safe mode in the electric power steering apparatus.